The present invention relates to a novel substituted lithium-manganese metal phosphate, a process for producing it as well as its use as cathode material in a secondary lithium-ion battery.
Since the publications by Goodenough et al. (J. Electrochem. Soc., 144, 1188-1194, 1997) there has been significant interest in particular in using lithium iron phosphate as cathode material in rechargeable secondary lithium-ion batteries. Lithium iron phosphate, compared with conventional lithium compounds based on spinels or layered oxides, such as lithium manganese oxide, lithium cobalt oxide and lithium nickel oxide, offers higher safety properties in the delithiated state such as are required in particular for the use of batteries in future in electric cars, electrically powered tools etc.
Pure lithium iron phosphate material was improved by so-called “carbon coating” (Ravet et al., Meeting of Electrochemical Society, Honolulu, 17-31 Oct. 1999, EP 1 049 182 A2), as an increased capacity of the carbon-coated material is achieved at room temperature (160 mAH/g).
In addition to customary solid-state syntheses (U.S. Pat. No. 5,910,382 C1 or U.S. Pat. No. 6,514,640 C1), a hydrothermal synthesis for lithium iron phosphate with the possibility of controlling the size and morphology of the lithium iron phosphate particles was disclosed in WO 2005/051840.
A disadvantage of lithium iron phosphate is in particular its redox couple Fe3+/Fe2+ which has a much lower redox potential vis-à-vis Li/Li+ (3.45 V versus Li/Li+) than for example the redox couple Co3+/Co4+ in LiCoO2 (3.9 V versus Li/Li+).
In particular lithium manganese phosphate LiMnPO4 is of interest in view of its higher Mn2+/Mn3+ redox couple (4.1 volt) versus Li/Li+. LiMnPO4 was also already disclosed by Goodenough et al., U.S. Pat. No. 5,910,382.
However, the production of electrochemically active and in particular carbon-coated LiMnPO4 has proved very difficult.
The electrical properties of lithium manganese phosphate were improved by iron substitution of the manganese sites:
Herle et al. in Nature Materials, Vol. 3, pp. 147-151 (2004) describe lithium-iron and lithium-nickel phosphates doped with zirconium. Morgan et al. describes in Electrochem. Solid State Lett. 7 (2), A30-A32 (2004) the intrinsic lithium-ion conductivity in LixMPO4 (M=Mn, Fe, Co, Ni) olivines. Yamada et al. in Chem. Mater. 18, pp. 804-813, 2004 deal with the electrochemical, magnetic and structural features of Lix(MnyFe1-y)PO4, which are also disclosed e.g. in WO2009/009758. Structural variations of Lix(MnyFe1-y)PO4, i.e. of the lithiophilite-triphylite series, were described by Losey et al. The Canadian Mineralogist, Vol. 42, pp. 1105-1115 (2004). The practical effects of the latter investigations in respect of the diffusion mechanism of deintercalation in Lix(MnyFe1-y)PO4 cathode material are found in Molenda et al. Solid State Ionics 177, 2617-2624 (2006).
However, a plateau-like region occurs for the discharge curves at 3.5 volt vis-à-vis lithium (iron plateau), the length of which compared with pure LiMnPO4 increases as the iron content increases, which results in a loss of energy density (see Yamada et al. in the publication mentioned above). The slow kinetics (charge and discharge kinetics) of manganese-containing metal phosphates, in particular Lix(MnyFe1-y)PO4 with y>0.8, have so far made the use of these compounds for battery applications largely impossible.